A Thin Film Bulk Acoustic Resonator (FBAR or TFBAR) is a device consisting of a piezoelectric material sandwiched between two electrodes and acoustically isolated from the surrounding medium. FBAR devices using piezoelectric films can be used as radio frequency (RF) filters for use in cell phones and other wireless applications. FBARs can also be used in a variety of other applications such as microwave oscillators and sensor applications.
FBARs utilize the acoustic resonant characteristics of piezoelectric materials, such as AlN or ZnO, to remove unwanted frequencies from being transmitted in a device, while allowing other specific frequencies to be received and transmitted. The resonant frequency is inversely proportional to the thickness of the piezoelectric film. Accordingly, to adjust the acoustic resonant characteristics of the FBAR, thickness of the piezoelectric film can be adjusted. Alternatively or in addition, the FBAR can be put under a mechanical load so that its resonance frequency can shift. This is known as a mass loading effect.
The mass loading effect can be accomplished by depositing or growing a mass of film on the resonator to bring about downshifting of the resonance frequency of the FBAR. The mass loading is carried out by growing or depositing of a thin film material uniformly distributed on one electrode of the resonator, covering the active surface of the device. The thin film can be a dielectric material or a metal material, depending on the design criteria.
However, it is known that processing flows may affect the acoustic resonant characteristics of the FBAR. For example, more or less piezoelectric material can be deposited for devices in different runs, due to processing variations. Similarly, more or less mass loading material can be deposited for devices in different runs, due to processing variations. Moreover, once the FBAR is built, it is difficult, if not impossible, to tune the resonant frequency.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.